SubSift: a novel application of the vector space model to support the

SubSift: a novel application of the vector space model to support the
JMLR: Workshop and Conference Proceedings 11 (2010) 20–27
Workshop on Applications of Pattern Analysis
SubSift: a novel application of the vector space model to
support the academic research process
Simon Price
[email protected]
Institute for Learning and Research Technology,
University of Bristol,
Bristol BS8 1HH, UK
Peter A. Flach
Sebastian Spiegler
[email protected]
[email protected]
Department of Computer Science,
University of Bristol,
Bristol BS8 1UB, UK
Editors: Tom Diethe, Nello Cristianini, John Shawe-Taylor
SubSift matches submitted conference or journal papers to potential peer reviewers based
on the similarity between the paper’s abstract and the reviewer’s publications as found in
online bibliographic databases such as Google Scholar. Using concepts from information
retrieval including a bag-of-words representation and cosine similarity, the SubSift tools
were originally created to streamline the peer review process for the ACM SIGKDD’09
data mining conference. This paper describes how these tools were subsequently developed
and deployed in the form of web services designed to support not only peer review but also
personalised data discovery and mashups. SubSift has already been used by several major
data mining conferences and interesting applications in other fields are now emerging.
Keywords: vector space model, cosine similarity, web services
1. Introduction
In this paper we describe how SubSift, a novel application of concepts from information
retrieval to problems in conference paper peer review, has given rise to a more general,
re-usable set of web services with interesting applications in other domains, for instance
profiling research groups and organisations. We begin in this section with an overview of
the development of the original software and its subsequent redevelopment and deployment.
1.1. Motivation and implementation of the original SubSift software
Peer review of written works is an essential pillar of the academic process, providing the
central quality control and feedback mechanism for submissions to conferences, journals
and funding bodies across a wide range of disciplines. However, from the perspective of a
busy conference chair, journal editor or funding manager, identifying the most appropriate
reviewer for a given submission is a non-trivial and time-consuming task. Effective assignment, first and foremost, requires a good match to be made between the subject of the
© 2010 S. Price, P.A. Flach & S. Spiegler.
submission and the corresponding expertise of reviewers drawn from a, sometimes large,
pool of potential reviewers. In the case of conferences, a recent trend transfers much of
this allocation work to the reviewers themselves, giving them access to the full range of
submissions and asking them to bid on submissions they would like to review. Their bids
are then compared to inform the allocation decisions of the programme committee chair.
It was the challenge of just such a bidding process that motivated the second author of
this paper, programme co-chair for ACM SIGKDD International Conference on Knowledge
Discovery and Data Mining 2009 (SIGKDD’09), to develop the original SubSift tools – not
as research per se, but as a practical implementation of theory (Flach et al., 2009).
SubSift, short for submission sifting, addressed three specific problems: (i) matching submissions to reviewers; (ii) ranking potential assignments; (iii) allocating papers to reviewers.
In the first step, each reviewer’s bids were initialised based on textual similarity between the
paper’s abstract and the reviewer’s publication titles, as listed in the DBLP bibliographic
database. In the second step, each of the 199 reviewers was sent an email containing a link
to a personalised SubSift generated web page listing details of all 537 papers ordered by initial bid allocation and similarity to their own published works. Guided by this personalised
perspective, plus the usual titles and abstracts, reviewers affirmed or revised their bids. In
the final step, after all reviewer bids were submitted the programme co-chairs were able
to consult, for any paper, a similarity ranked list of reviewers to assist them in allocating
papers with either too few or too many bids1 .
To quantitatively evaluate SubSift’s performance, we compared SubSift’s initial bids
against the revised final bids made by SIGKDD’09 reviewers. Disregarding the level of
the bid, we calculated: precision as the proportion of non-zero actual bids among the nonzero initial bids; recall as the proportion of non-zero initial bids among the non-zero actual
bids; and F-measure as the harmonic mean of precision and recall. Over all reviewers the
median F-measure is 72.7%; median precision 88.2%; and median recall 80.0% 2 . These
results, plus qualitative feedback from reviewers, were encouraging and demonstrated that
there is considerable scope for automated support during the bidding process.
1.2. Subsequent redevelopment and deployment as web services
The application of the original SubSift tools for SIGKDD’09 prompted requests from programme committee (PC) chairs of other data mining conferences to re-use the software in
other settings. This in turn prompted a successful bid to the UK Joint Information Services
Council (JISC) for a software development project to repackage SubSift tools as SubSift
Services, a collection of lightweight web services, usable individually or in mashups, to support: matching reviewers with submissions, ranking potential assignments, and allocating
papers to reviewers.
One the key goals of the SubSift Services project is to share and enable re-use of the
software in different settings, and in a sustainable way. The decision to repackage and
host SubSift as web services was in direct pursuit of this goal. Apart from the obvious
advantage of prospective users being able to use the software without having to install it,
1. Beyond the scope of this article, the KDD’09 tools also assisted in the constraint satisfaction problem of
allocating papers given reviewer bids and conflicts of interest, and of calibrating reviewer’s scoring.
2. We use the median because of the skew of the distribution.
Price Flach Spiegler
the web services model has a number of other benefits, including the ability for users to:
make their own application-specific customisations by integrating the services with mashup
tools like Yahoo Pipes; select other data sources, such as Citeseer, Google Scholar, eprints,
news and blogs; and, importantly, integrate the software with their own research outputs
without having to modify code. However, in recognition that web services will not suit
all applications, the SubSift Services project has released the software as Open Source on
Google Code. Further details of SubSift are available on the project website3 .
To demonstrate SubSift Services the project produced an example workflow as a series
of web forms that: (i) accepts a list of author names, looks up these names on the DBLP
Computer Science Bibliography4 and suggests author pages which, after disambiguation,
are used to profile the reviewers based on the text of these pages; (ii) allows the user to
upload a CSV file containing all the submitted paper abstracts, which form the basis of the
abstract profiles; and (iii) compares reviewer profiles to paper abstract profiles, producing
downloadable web pages with ranked lists of papers per reviewer, and ranked lists of reviewers per paper. Although only a demonstrator, this workflow has now been used to support
several major data mining conferences.
The remainder of this paper begins with a review of relevant background topics, followed
by a tour of the SubSift Services API, and a look at some interesting applications in other
areas, before rounding up with future work and concluding remarks.
2. Background
The theoretical basis for the text matching component of SubSift, and of SubSift Services, is
the well known vector space model from information retrieval (Salton et al., 1975). SubSift
Services software also draws on the representational state transfer (REST) design pattern
for web services (Fielding, 2000). In this section we give a brief overview of each of these
topics. Readers already familiar with them can safely skip to the next section.
2.1. Vector Space Model
The canonical task in information retrieval is, given a query in the form of a list of words
(terms), rank a set of text documents D in order of their similarity to the query. The
vector space model is a common approach to solving this problem. Each document d ∈ D
is represented as the multiset of terms (bag-of-words) occurring in that document. The set
of distinct terms in D, vocabulary V , defines a vector space with dimensionality |V | and
thus each document d is represented as a vector d in this space. The query q can also be
represented as a vector q in this space, assuming it shares vocabulary V . The query and
a document are considered similar if the angle θ between their vectors is small. The angle
can be conveniently captured by its cosine, which is equal to the dot product of the vectors
scaled to unit length, giving rise to the cosine similarity, s.
3. SubSift –
4. DBLP Computer Science Bibliography –
s(q, d) = cos(θ) =
||q|| · ||d||
However, if raw term counts are used in vectors q and d then similarity will be biased in
favour of long documents and will treat all terms as equally important. The term frequency
– inverse document frequency (tf-idf) weighting scheme compensates for this by normalising
term counts within a document by the total number of terms in that document, and by
penalising terms which occur in many documents. More formally, term frequency tfij of
term ti in the document dj , and inverse document frequency idfi of term ti are defined as
tfij = P
tf-idfij = tfij × idfj
idfi = log2
k nkj
where term count nij is the number of times term ti occurs in the document dj , document
frequency dfi of term ti is the number of documents in D in which term ti occurs.
In SubSift, instead of comparing a single query against a set of documents, we pairwise compare every document in one collection D1 (e.g. abstracts) with every document
in another collection D2 (e.g. reviewer bibliographies) to produce a ranked list for each
document. To capture the overall importance of each term across the combined collections,
dfi , and hence tf-idfi , values are calculated over the union of both collections, D1 ∪ D2 .
2.2. Representational State Transfer (REST)
REST is an easily understood design pattern for web services, based around the ubiquitous
HTTP protocol and its familiar vocabulary of URIs, media types, requests and responses
(Fielding, 2000; Fielding and Taylor, 2002). Recently, REST web services have become
popular as the web API behind numerous Web 2.0 sites, including Twitter, Flickr and
Facebook. Like conventional websites, RESTful websites offer a stateless, cacheable, layered
and uniform client-server interface. However, unlike conventional sites, which are designed
to render human-readable data as HTML pages to be viewed in a browser, RESTful sites
serve data in formats such as XML, JSON and CSV, that may be readily consumed by
arbitrary applications. Furthermore, in the same way that HTML forms on conventional
web pages can be used to submit data from the client to the server for storage and processing,
arbitrary RESTful applications can use exactly the same protocols to achieve the same end.
Also, the usual HTTP authentication and authorisation mechanisms can be used to control
access to specific services and resources.
The intuition behind REST is that URIs are used to represent resources and that HTTP
request methods are used to specify fundamental operations on those resources. Additional
operations are specified by adding verbs into the URIs. The most widely used HTTP request
method, GET, is invoked every time a web browser requests a URI. This usually returns a
web page in the form of an HTTP response which the browser displays. However, GET is
only one of several HTTP request methods; the five that are most significant for REST web
services are summarised in the table below.
Price Flach Spiegler
HTTP Method
Usage in REST
show and list operations
exists operations to check if a resource exists
create and compute operations
update and recompute operations
destroy operations
Changes Resource
Of these, the most familiar to HTML authors are GET and POST - as used in hyperlinks (e.g. <a href="">...</a>) and HTML forms (e.g. <form
method="POST" action="...">...</form>). As is the case with HTML forms, pairs of
attribute-value parameters can be supplied with the HTTP request in the usual way.
3. SubSift REST API
The SubSift REST API is organised around a series of folders into which data items are
stored. This organisation is modelled on the familiar filing system concept of folders and
files. The three main folder types are documents, profiles and matches. In SubSift, a
document is a piece of text to be profiled and matched. A document will usually be the text
from some external source such as the text of a web page or a conference paper abstract. A
profile is a summary representation of the features of a single document, with respect to the
other documents in the same documents folder. One usage of profiles in SubSift is to obtain
a list of distinguishing terms, or keywords, for a document – for example, automatically
extracting keywords from abstracts of papers submitted to a conference. Another usage is
for two profile folders to be compared against each other to produce a matches folder. A
matches folder is created by analysing every pairing of profile items drawn from the two
profiles folders. Each match item records the tf-idf cosine similarity, and various related
statistics, of a single profile from the first profiles folder against every profile from the second
profiles folder. A typical usage of such a comparison is to match submitted conference
abstracts with the bibliography pages of programme committee members in order to rank
potential reviewers for each paper and visa versa. This usage is depicted in Figure 1.
A wide range of API methods make intermediate data and metadata available for every
type of folder and item. For example, the relative contribution of each term towards a
particular similarity calculation can be retrieved or the entire similarity matrix exported.
For ease of integration, there is flexibility in both input and output. Document text may
be added per item or in bulk or by supplying a list of URLs to be fetched asynchronously
by SubSift’s harvester robot. The API methods can return data in the following representational formats: CSV, JSON, XML, YAML, and Prolog terms.
4. Applications in Other Areas
In this section we outline an exploratory investigation of a potential application of SubSift
to social network analysis within an organisation, and briefly mention other uses that have
emerged. Figure 2 shows a fragment of a dendrogram produced in Matlab by clustering
a similarity matrix calculated by SubSift, pairwise matching ILRT staff homepages. The
numbers to the right of the labels represent the staff member’s actual ILRT project group
Figure 1: A SubSift REST API
controlled sequence of
transformations from a
pair of document folders (e.g.
versus PC members)
through to a folder
of matching statistics.
Useful data and metadata can be obtained at
each step in the process
via API methods.
ilrt staff x ilrt staff (homepages)
Figure 2: Using SubSift similarity data to cluster ILRT staff homepages.
Price Flach Spiegler
membership, and the chart on the right shows pairwise precision and recall for thresholds
at each node in the dendrogram. While the precision-recall plot suggests no single ideal
threshold, some project groups are rediscovered by the clustering but, more interestingly,
the clustering also reveals older project groupings and co-authorships prior to organisational
restructuring at ILRT. Potential applications might include locating research bid partners
across an organisation or identifying informal structures within an organisation.
Other applications include using SubSift to produce a “profiling” list of keywords associated with a person or a group of people, followed by transformation into Friend of a
Friend (FOAF) semantic web metadata or, more visually, into tag cloud diagrams. Using
the ranked lists from matches allows the creation of network diagrams visualising social
networks and, along similar lines, following friends’ interests on Twitter. SubSift is also
being used to produce research datasets for machine learning experiments.
5. Future Work
Although SubSift Services is neither pure research nor pure IT, there are potential developments in both these areas. On the research administration front, SubSift is a useful
service for constructing, manipulating and publishing document-centric data sets, and the
REST model itself is a promising way to persist and share algorithms within the research
community. SubSift, by its very nature, has potential for integration into a wide range of
applications ranging far beyond the original peer review domain.
We are now building on the SubSift tools to develop ExaMiner, an application for mining
and mapping a university’s research landscape, beginning initially with the University of
Bristol but with the ultimate aim of being able to map any research-centric institution.
Specifically, the ExaMiner software will: (i) generate automatic profiles of researchers and
research groups from university web pages; (ii) allow visual navigation of these profiles, as
well as navigation by free-text query; (iii) provide social networking components, including
the ability to store a personal profile and a network of interesting contacts, events and
connections; (iv) notify a user of new resources in their sphere of interest; and (v) improve
performance over time by responding to user feedback. Possible users include academics,
research managers, current and prospective students, and the media. The ExaMiner project
has been funded by the University of Bristol and it is hoped that a system based on this prototype will improve the discoverability of its research within the institution, the community,
the media, prospective students, and the government.
6. Concluding Remarks
The implementation of SubSift Services as a repackaged version of the text matching functionality of the SIGKDD’09 software has created a more general purpose resource with potential applications in areas outside the specific domain of peer review. A similar approach
may be successful in publishing the functionality of other research-produced applications.
We would like to thank conference PC chairs Bart Goethals (SDM’10), Qiang Yang (KDD’10),
Mohammed Zaki (PAKDD’10) and Geoff Webb (ICDM’10) for trialing SubSift; the SubSift Services project team members Nicola Rogers and Chris Bailey from ILRT; and other
developers of the original software, Bruno Golénia from Department of Computer Science,
John Guiver, Ralf Herbrich and Thore Graepel from Microsoft Research in Cambridge, and
Mohammed Zaki from Rensselaer Polytechnic Institute.
Roy T. Fielding and Richard N. Taylor. Principled design of the modern web architecture.
ACM Transactions on Internet Technology, 2:115–150, May 2002. ISSN 1533-5399. doi:
Roy Thomas Fielding. REST: Architectural Styles and the Design of Network-based Software
Architectures. Doctoral dissertation, University of California, Irvine, 2000. URL http:
Peter A. Flach, Sebastian Spiegler, Bruno Golénia, Simon Price, John Guiver Ralf, Herbrich Thore Graepel, and Mohammed J. Zaki. Novel tools to streamline the conference
review process: Experiences from SIGKDD’09. SIGKDD Explorations, 11(2):63–67, December 2009. ISSN 1931-0145. doi:
G. Salton, A. Wong, and C. S. Yang. A vector space model for automatic indexing. Commun.
ACM, 18(11):613–620, 1975. ISSN 0001-0782. doi:
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